1. Field of the Invention
The present invention relates to a so-called direct-plate-making IR laser-sensitive planographic printing plate precursor that allows direct plate-making on the basis of digital signals for example from computer, and in particular to a planographic printing plate precursor superior in chemical resistance, greater in solubility change in the exposed area, and superior in image reproducibility.
2. Description of the Related Art
Planographic printing plates that allow plate making with infrared laser are attracting attention recently as planographic printing plates. In the recent rapid progress in laser, higher-output and smaller solid state and semiconductor lasers having an emission wavelength in the range from near-infrared to infrared are becoming more easily accessible. These lasers play an important role as an exposure-light source, when planographic printing plates are produced directly form digital data, for example, from computer.
Materials which can be used for a positive type planographic printing plate precursors applicable for infrared lasers include, as essential components, a binder resin soluble in an aqueous alkaline solution and an infra red dye which absorbs light to generate heat. The infra red dye interacts with the binder resin in an unexposed portions (image portions) so as to function as a dissolution inhibitor which can substantially reduce the solubility of the binder resin. On the other hand, in an exposed portions (non-image portions), interaction of the infra red dye with the binder resin is weakened by the heat generated. Consequently, an exposed portion can turn into a state in which it can be dissolved in an alkaline developer, so that an image is formed thereon.
However, insofar as infrared-laser-applicable positive planographic printing plate precursor materials are concerned, differences in the degree of resistance against dissolution in a developer between unexposed portions (image portions) and exposed portions (non-image portions) therein, that is, differences in development latitude have not yet been sufficient under various conditions of use. Thus, problems have occurred insofar that, with changes in developing conditions, the unexposed portions (i.e., the image portions) may be dissolved during development or stains at the non-image portion due to insufficient development may be generated.
Such problems stem from fundamental differences in plate-making mechanisms between infrared-laser-applicable positive type planographic printing plate precursor materials and positive type planographic printing plate precursor materials from which printing plates are made up by exposure to ultra violet rays.
In other words, positive type planographic printing plate precursor materials from which printing plates are made up by exposure to ultra violer rays each include, as essential components, a binder resin soluble in an aqueous alkaline solution and an onium salt, or a quinonediazide compound. This onium salt or quinonediazide compound not only interacts with the binder resin in unexposed portions (image portions) to function as a dissolution inhibitor, but in exposed portions (non-image portions) it is also decomposed by light and generates an acid to function as a dissolution promoter. In this way, the onium salt, or the quinonediazide compound, performs dual functions.
On the other hand, in infrared-laser-applicable positive type planographic printing plate precursor materials, the infra red dye functions only as a dissolution inhibitor of unexposed portions (image portions), and does not promote the dissolution of exposed portions (non-image portions).
In addition, the image-forming efficiency of such infrared-laser positive-type planographic printing plate precursors depends on the heat generated as a result of exposure of the recording layer surface to infrared laser. The amount of heat for use in image forming, i.e., the amount of heat used for solubilization of the recording layer, is smaller in the region closer to the support due to diffusion of the heat to the support, making the planographic printing plate precursor less sensitive. Accordingly, reduction of the development-suppressing effect at a non-image region of the recording layer is not sufficient, leading to decrease in difference between image and non-image regions and consequently a problem of insufficient image reproducibility.
A recording layer formed by using a material that can be easily developed in the non-image region may be effective in solving the aforementioned problem in image reproducibility, but the image region of such a recording layer exhibits poor chemical resistance, i.e., is chemically weak and vulnerable to the developer and the ink cleaning solvent, plate cleaner, and the like used during printing. Thus, there existed an urgent need for a recording layer that is superior in the chemical resistance and durability of the film and superior in developability after release or cancellation of the dissolution-suppressing effect.
A method of forming a recording layer by using a polymer having a maleimide group that is superior in developability and chemical resistance as the binder has been proposed to solve the problems above, (e.g., Japanese Patent Application National Publication (Laid-Open) No. 2002-517786). However, although the recording layer of JP A 2002-517786 was improved in chemical resistance to some extent, the dissolution-suppressing effect in the unexposed region is not sufficient, causing problems such as: generation of white blank in an image region due to undesirable dissolution of the region during development which is facilitated by small scratches formed around the region before development; and undesirable decrease in the image area due to dissolution of micro-area images such as dots and thin line from the side faces thereof during development.
Alternatively, a method of improving the solubilization resistance to alkali developers in the area closer to the surface, by introducing a polar group such as water into the recording layer from the surface, has been studied [e.g., Japanese Patent Application Laid-Open (JP-A) No. 2001-133965]. However, although the method was effective in improving the sensitivity and chemical resistance to some extent, such improvements are not satisfactory, and in particular, chemical resistance is far from the satisfactory level. Further, developability is insufficient in the deeper region of the recording layer closer to the support, causing insufficient solubilization discrimination (difference in solubility between exposed and unexposed regions). Therefore, further improvement in image reproducibility has been desired.